The present application is based on and claims priority from Japanese Patent Application Hei 2000-147754 filed May 18, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an oil-impregnated sintered bearing that has a non-porous slide member.
2. Description of the Related Art
Usually, an oil-impregnated sintered bearing is apt to be short of an oil film formed between a rotating shaft of a device and the slide surface of bearing when load applied to the shaft increases to a certain level. Shortage of the oil film may lower not only the performance of the device but also the lifetime thereof.
JP-Y2-46-32487 discloses a bearing in which an abrasion resistant resin is impregnated into the inner peripheral portion of a porous metal body member to form a non-porous slide surface.
However, there is a big difference (e.g. as big as 10 times) in the coefficient of thermal expansion between such an abrasion resistant resin and the metal body member. If such a bearing is used at a temperature that is lower than the temperature when the resin was impregnated, the impregnated abrasion-resistant resin contracts. This reduces the inside diameter of the slide surface and, consequently, the clearance between the slide surface and the shaft. As a result, the shaft can not rotate smoothly, and the lifetime of the bearing may shorten.
JP-B2-2685333 discloses a bearing, in which a non-porous metal member is inserted into a cylindrical sintered bearing body to form a slide surface. Although there is a small difference in the coefficient of thermal expansion between the shaft and the slide surface, there is a considerable friction resistance between the two members because of a metal-to-metal contact when an oil film is not formed under a low temperature. This friction also reduces the rotation efficiency and the lifetime of the bearing.
Therefore, a main object of the invention is to provide an improved oil-impregnated sintered bearing that has a long lifetime and high rotation efficiency.
An oil-impregnated sintered bearing according to an embodiment of the invention includes a cylindrical porous bearing body having a first portion providing a part of the slide surface and a second portion having an inner periphery and a non-porous slide member disposed at the inner periphery of the second portion providing the other part of the slide surface. The slide member has a radial thickness of about 20% of the radial thickness of said bearing.
The slide member can be formed by a resinous material that is impregnated into the second portion of the bearing body. Therefore, the slide member is securely fixed to the bearing body and will not peel off.
On the other hand, the second portion of the bearing body can provide a recess in which the slide member is fixedly disposed. Because the bonding force is comparatively weaker, the inside diameter of the bearing is not affected by the contraction of the slide member under a low temperature.
The slide surface of the bearing can include a cylindrical middle slide surface and cylindrical axial end surfaces gradually expanding in opposite directions as the end surfaces leave from the middle surface. Even if a shaft inserted into the bearing is inclined relative to the axis of the bearing, the shaft can be properly supported by one of the end surfaces, thereby increasing the lifetime of the bearing.
Preferably, the bearing body has a mark for indicating the slide member.
It is also preferable that the slide member is made of fluororesin. However, the slide member can include graphite and/or molybdenum powder.